Radiotherapy is a treatment for cancer patients involving the use of high-energy radiation. When high-energy radiation is delivered to a subject, it kills cells in the body. Although the high-energy radiation kills tumor cells in the subject's body, it may also kill normal tissue cells and tissue cells of an organ-at-risk (OAR) that surround the tumor. Thus, the goal of conventional radiotherapy is to deliver a sufficient radiation dose to the tumor to kill the tumor cells while minimizing the radiation dose delivered to the normal tissue cells and OAR tissue cells that surround the tumor.